Colloids Flashcards
Why colloidal particle are like ‘big atoms’?
3 points
- Easy to see
- Not limited by QM
- Can tune interractions between them
Lyophobic colloids
+ Example
Dispersed particles do not like the suspended medium (emulsions)
Lyophillic colloids
+ Example
Suspended particles like the solvent (gelatin sol)
Describe Tyndall scattering
4 points
- Light gets scattered by suspended particles
- Particle size is similar to visible light wavelength
- Blue light gets scaterred more (\propto 1/lambda^4)
- Red light gets transmitted
Floc
Loose agglomeration of particles to form an open aggregate. Reversible aggregation
Coagulum
A strong agglomeration of particles to form a dense aggregare. Irreversible aggregation
Sedimentation
Aggregate of colloidal particles denser than solvent
Creaming
Aggregate of colloidal particles less denser than solvent
Polydispercity
Distribution of particle size. Standard dev over mean
Electrical double layer
Comnination of Stern layer and the diffuse layer. These screen the electrostatic interractions experience by the charged surface
Stern layer
Oppositely chatged ions strongly adsorbed on a charged surface
Diffuse layer
The layer aftern Stern layer where there are more oppositely charged ions than in the bulk sollution
Critical coagulation concentration
Salt concentration at which ΔG* = 0
What happends at medium salt concentration?
Particles achieve a free energy minimum (floc)
What happends at low salt concentration?
Repulsions are not screened by salt in-between
Energy barriers is big => stable colloid
What happends at high salt concentration?
3 points
- Short range repulsions
- Electrostatic repulsions are screened by the salt
- VDW wins and substances coagulated irreversibly
Surfactor
Emulsification agent
Describe the suspension method
2 points
- Mix two immiscibe liquids + surfactor
- Surfactor sits at the interfaces and stabilises the liquid
Suspension method
What else can be used to stabilise the suspension?
Other colloidal particles
Pickering emulsion
When emulsification agent consists of solid particles
Define condensation method
Formation of colloidal particles from molecules up
3 stages of condensation method
- Nucleation
- Growth
- Stabilisation
What does the classical nucleation theory suggest?
4 points
- Solid like particles suddenly appear
- Outcome depends on Gibbs free energy of formation of the particle
- Free energy depends on particle size
- Particle redissolves if its radius smaller than the critical radius
Ostwald ripening
When too many things have nucleated, smaller particles redissolve to form bigger particles
Describe ouzo effect
Formation of milky oil in water emulsion upon addition of water to some liqueurs
Explain ouzo effect
- Oil in water emulsions are not stable
- Oil droplets coalesce until complete phase separation is achieved (at macroscopic levels)
- Addition of small amounts of surfactant stabilises the oil droplets
What are the 2 effects giving an overall static equilibrium for a colloidal system?
- Brownian motion
- Gravity
How can the colloidal equilibrium be described?
Boltzman’s distribution
What happens to attraction strength as [polymer] increases
Increases
What happens to attraction range as coil diameter of polymer increases
Increases
What does the presence of polymer in a colloidal mixture result in? Why?
- Attraction between the colloidal particles
- Entropy & Osmotic pressure
Define
Critical supersaturation
The concentration required to promote the spontaneous formation and growth of solid particles
Explain why the nucleation rate and final particle size are sensitive to the
supersaturation.
Increasing the supersaturation lowers the barrier and exponentially increases the rate of nucleation. Faster nucleation rate leads to smaller particle formation
If supersaturation (S) is increased from 1.01 to 1.02 what happens to critical radius and deltaG barrier
Critical cluster radius is reduced by a factor of 2, and the barrier height is reduced by a factor
of 4.
